The present invention relates generally to a projection optical system and an exposure apparatus having the same, and more particularly to a catoptric projection optical system, an exposure apparatus, and a device manufacturing method, which utilizes the extreme ultraviolet (“EUV”) as a light source to expose a plate, such as a single crystal substrate for a semiconductor wafer, and a glass plate for a liquid crystal display (“LCD”).
Along with recent demands for smaller and lower profile electronic devices, the fine processing to semiconductor devices to be mounted onto these electronic devices has been increasingly demanded. For the demand of the fine processing to the semiconductor device, a reduction to practice of a reduction projection exposure apparatus using EUV light with a wavelength less than 20 nm (such as about 13.5 nm) shorter than that of the UV light (referred to as an “EUV exposure apparatus” hereinafter) has been studied. Since there is no usable glass material for the EUV light as the exposure light, and a catoptric projection optical system is proposed which includes only mirrors (e.g., multilayer mirrors).
The smaller number of mirrors is preferable for the catoptric projection optical system to enhance the reflectance of the entire optical system. In addition, the even number of mirrors is preferable so that the mask and the wafer oppose to each other with respect to the pupil to prevent mechanical interference between the mask and wafer. In addition, as the critical dimension (“CD”) or resolution required for the EUV exposure apparatus has been smaller than the conventional value (for example, currently 32 nm node is required), a numerical aperture (“NA”) at the image side should be increased, but it is difficult four three or four mirrors to reduce the wavefront aberration. Accordingly, it is preferable that the number of mirrors is made six so as to increase the degree of freedom to correct the wavefront aberration. See, for example, U.S. Pat. No. 6,033,079 and Japanese Patent Application, Publication No. 2003-15040.
U.S. Pat. No. 6,033,079 discloses a typical projection optical system that includes six mirrors for two EUV lights. The projection optical system receives the incident light form the object plane, and forms an intermediate image via a first concave reflective surface, a second concave or convex reflective surface, or a third convex reflective surface. The projection optical system re-images the intermediate image on the image plane via a fifth convex reflective surface, and a sixth concave reflective surface. The projection optical system provides an aperture stop on the second reflective surface.
Japanese Patent Application, Publication No. 2003-15040 discloses a typical projection optical system that includes six mirrors for two EUV lights. The projection optical system receives the incident light form the object plane, and forms an intermediate image via a first concave reflective surface, a second convex reflective surface, a third convex reflective surface, and a fourth concave reflective surface. The projection optical system re-images the intermediate image on the image plane via a fifth convex reflective surface, and a sixth concave reflective surface. The projection optical system provides an aperture stop between the second and third reflective surfaces.
Other prior art that disclose similar optical systems are Japanese Patent Applications, Publication Nos. 2004-22722, 2004-170869, 2004-138926, 2000-235144, 2001-185480, 2004-31808, 2004-138926, and 2004-258178, and U.S. Pat. Nos. 6,199,991 and 6,255,661.
However, the first embodiment described in U.S. Pat. No. 6,033,079 has a problem of a plane interval or separation. Le1/TT=0.4201, Le2/TT=0.4202, and Le3/TT=0.4686 are met. Since these values are approximately the same, there occur problems in that the front focus is short and the arrangement of components is difficult. Here, TT is an absolute value between the object and image. Le1 is an absolute value of the interval between the plane apexes of the first and second reflective surfaces on the optical axis. Le2 is an absolute value of the interval between the plane apexes of the second and third reflective surfaces on the optical axis. Le3 is an absolute value of the interval between the plane apexes of the third and fourth reflective surfaces on the optical axis.
The first embodiment in Japanese Patent Application, Publication No. 2003-15040 has a problem of a light incident angle upon the surface. On the predetermined plane, angle θ11 is 25.83° between the first reflective surface and the principal ray that is existed from the object point at the center of an arc-shaped illuminated area on the object plane. Angle θ21 is 39.35° between the principal ray and the second reflective surface. Angle θ31 is 35.44° between the principal ray and the third reflective surface. Thus, values of θ21 and θ31 are large. The sixth mirror M6 has a large effective diameter for a high NA, and the fourth mirror M4 is located apart from the optical axis to introduce the light to the fifth mirror M5 to avoid the sixth mirror M6. In order to introduce the light from the object to M4, the incident angles at the second and third mirrors become very large and unnaturally deflect the light, thereby posing problems of a generation of an aberration and a lowered reflectance due to the influence of the multilayer coating. As a result, this configuration cannot provide highly precise exposure or lowers the throughput due to the decreased light intensity.